In situ annealing procedures are to be used with I125-RNA to identify the sites of the S5 ribosomal RNA cistrons in several species of Drosophila. The S5 gene placement will be compared to the location of the nucleolus organizers. Amphibian lampbrush chromosomes as well as male meiotic material are to be used with I125-RNA to map the position of the S5 and 18-28S rRNA genes on these large chromosomes. A few highly purified transfer RNAs are to be iodinated and annealed to the polytene chromosomes of Drosophila melanogaster in order to identify the transfer RNA gene locations.